A solid electrolytic capacitor generally consists of a porous metal electrode, an oxide layer located on the metal surface, an electrically conductive solid introduced into the porous structure, an outer electrode, such as a silver layer, and further electric contacts and encapsulation.
Examples of solid electrolytic capacitors are tantalum, aluminium, niobium and niobium oxide capacitors with charge transfer complexes, manganese dioxide or polymeric solid electrolytes. The use of porous bodies has the advantage that very high capacity densities (i.e. high capacitance) may be achieved in a small space owing to the large surface area.
π-conjugated polymers are particularly suitable as solid electrolytes owing to their high electrical conductivity. π-conjugated polymers are also called conductive polymers or synthetic metals. They are becoming increasingly important economically, as polymers have advantages over metals, with respect to processability, weight and the targeted adjustment of properties by chemical modification. Polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes) are examples of known π-conjugated polymers, poly-3,4-(ethylene-1,2-dioxy)thiophene, often also called poly(3,4-ethylenedioxythiophene), being a particularly important, industrially used polythiophene, as it has very high conductivity in its oxidised form.
Technical development in electronics increasingly requires solid electrolytic capacitors with very low equivalent series resistance (ESR). This is due, for example, to decreasing logic voltages, a higher integration density and increasing clock frequencies in integrated circuits. A low ESR also reduces the energy consumption, and this is particularly advantageous for mobile, battery-operated applications. There is therefore a wish to reduce the ESR of solid electrolytic capacitors as far as possible.
European patent specification EP-A 340 512 describes the production of a solid electrolyte made of 3,4-ethylene-1,2-dioxythiophene and the use of its cationic polymer produced by oxidative polymerisation as the solid electrolyte in electrolytic capacitors. Poly(3,4-ethylenedioxythiophene) as a substitute for manganese dioxide or charge transfer complex in the solid electrolytic capacitors reduces the equivalent series resistance of the capacitor owing to the higher electrical conductivity and improves the frequency behaviour.
In addition to a low ESR, modern solid electrolytic capacitors require a low leakage current and good stability with respect to external stresses. High mechanical stresses that may greatly increase the leakage current of the capacitor anode occur in particular during the production process when encapsulating the capacitor anodes.
Stability toward such stresses and therefore a low leakage current may primarily be achieved by an approximately 5 to 50 μm thick outer layer made of conductive polymers on the capacitor anode. Such a layer is used as a mechanical buffer between the capacitor anode and the cathode-side electrode. This prevents the electrode, for example when mechanically stressed, from coming into direct contact with the anode or from damaging it and thus increasing the leakage current of the capacitor. The conductive polymeric outer layer itself exhibits what is known as self-healing behaviour: relatively small defects in the dielectric on the outer anode surface, which occur despite the buffer effect, are electrically insulated by the conductivity of the outer layer being destroyed at the defective point by the electric current.
The formation of a thick outer layer by in situ polymerisation is very difficult. Layer formation requires a very large number of coating cycles in this process. As a result of the large number of coating cycles, the outer layer is coated very unevenly, in particular the edges of the capacitor anode are often inadequately covered. Japanese patent application JP-A 2003-188052 recites that homogeneous edge coverage requires extensive matching of the processing parameters. However, this makes the production process very susceptible to interruptions. An addition of binder materials for quicker layer build-up is also difficult, as the binder materials hinder the oxidative in situ polymerisation. In addition, the layer polymerised in situ generally has to be freed from residual salts by washing, whereby holes are produced in the polymer layer.
A dense outer layer with good edge coverage may be achieved by electrochemical polymerisation. However, electrochemical polymerisation requires that firstly a conductive foil be deposited on the insulating oxide layer of the capacitor anode and that this layer is then electrically contacted for each individual capacitor. This contacting can be very complex in mass production and may damage the oxide layer.
The use of formulations containing the powder of a conductive polymer and binders have excessive electrical resistance owing to the high contact resistance between the individual powder particles, for them to allow production of solid electrolytic capacitors with low ESR.
In Japanese patent applications JP-A 2001-102255 and JP-A 2001-060535, a layer of polyethylenedioxythiophene/polystyrene sulphonic acid (PEDT/PSS), also called polyethylenedioxythiophene/polystyrene sulphonic acid complex or PEDT/PSS complex, is applied directly to the oxide film to protect the oxide film and for improved adhesion of the solid electrolyte to the oxide film. The outer layer is then applied to this layer by in situ polymerisation or by impregnation of the capacitor anode with tetracyanoquinodimethane salt solution. However, this process has the drawback that the PEDT/PSS complex does not penetrate into porous anode bodies with small pores. Consequently, modern, highly porous anode materials cannot be used.
U.S. Pat. No. 6,001,281 describes, in the examples, capacitors with a solid electrolyte made of polyethylenedioxythiophene (PEDT) produced in situ and an outer layer made of PEDT/PSS complex. However, the drawback of these capacitors is that they have a high ESR of 130 mΩ and higher.
There is therefore still a need for solid electrolytic capacitors with low equivalent series resistance (ESR), that have a dense polymeric outer layer with good edge coverage and a low leakage current. There is also still a need for a process for producing such capacitors.